And the 2015 Volt's efficiency remains the same as well, at 98 MPGe. (A Mile Per Gallon Equivalent, or MPGe, is the distance an electric vehicle can travel on the same amount of energy as contained in 1 gallon of gasoline.)

Kevin Kelly, GM's manager of electrification technology communications, confirmed the larger battery size to Green Car Reports. It was "a late change done in the cell," he said, terming it "just another continuous improvement to surprise and delight our customers."

Several Volt owners have suggested that while the rated range remains the same, 2015 Volt drivers may see a slight increase in real-world electric range over 2013 and 2014 models.

Kelly declined to discuss any potential second-generation Volt, noting that it was company policy not to comment on future vehicles.

Second battery boost

The 2015 Volt marks the second time Chevrolet has boosted the battery size in its range-extended electric car.

The Volt pack contained 16 kWh of energy in the original 2011 and 2012 models. For those two years, electric range was rated at 35 miles, and efficiency at 93 MPGe.

Other changes to the 2015 Volt are restricted to the availability of 4G LTE service in conjunction with GM's OnStar, as do many other Chevrolet vehicles for the 2015 model year.

The base price of the 2015 Chevy Volt remains at $34,995, including the mandatory delivery fee. The Volt is eligible for a $7,500 Federal income-tax credit for qualifying buyers, as well as a variety of state and local incentives.